Proposed resolution:

This wording is relative to N4861.

Change [time.duration.cons] as indicated:

template<class Rep2>
  constexpr explicit duration(const Rep2& r);

-1- Constraints: is_convertible_v<const Rep2&, rep> is true and

1. (1.1) — treat_as_floating_point_v<rep> is true or

2. (1.2) — treat_as_floating_point_v<Rep2> is false.

[Example: […] end example]

-?- Effects: Initializes rep_ with r.

-2- Postconditions: count() == static_cast<rep>(r).

[ 2020-05-02; Daniel resyncs wording with recent working draft ]

[ 2018-06-27 after reflector discussion ]

Priority set to 3. Improved wording as result of that discussion.

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.

Change [time.duration.cons] as indicated:

template<class Rep2>
  constexpr explicit duration(const Rep2& r);

-1- Remarks: This constructor shall not participate in overload resolution unless […]

-2- Effects: Constructs an object of type durationInitializes rep_ with r.

-3- Postconditions: count() == static_cast<rep>(r).

[time.duration]/4 states:

Members of duration shall not throw exceptions other than those thrown by the indicated operations on their representations.

Where representation is defined in the non-normative, brief description at [time.duration]/1:

[…] A duration has a representation which holds a count of ticks and a tick period. […]

[time.duration.cons]/2 doesn't indicate the operation undergone by its representation, merely stating a postcondition in [time.duration.cons]/3:

Effects: Constructs an object of type duration.

Postconditions: count() == static_cast<rep>(r).

I suggest this reformulation that follows the format of [time.duration.cons]/5.

Effects: Constructs an object of type duration, constructing rep_ from r.

Now it is clear why the constructor would throw.

Previous resolution [SUPERSEDED]:

This wording is relative to N4750.

Change [time.duration.cons] as indicated:

template<class Rep2>
  constexpr explicit duration(const Rep2& r);

-1- Remarks: This constructor shall not participate in overload resolution unless […]

-2- Effects: Constructs an object of type duration, constructing rep_ from r.

-3- Postconditions: count() == static_cast<rep>(r).